Electro-rheological or electro-viscous fluids are dispersions of finely divided solids in hydrophobic, electrically non-conductive oils wherein the viscosity of the fluid increases rapidly and reversibly from a low viscosity fluid to a highly viscous, plastic or solid state under the imposition of a sufficiently powerful electric field. Usually the increase in viscosity of the electro-viscous fluid is proportional to the applied voltage. The materials respond both to direct and alternating current, but it is required that there be very little flow of current through the electro-viscous fluid. These fluids are useful for the transmission of large forces while requiring only a very small amount of electric power. Devices employing electro-viscous fluids include among others clutches, hydraulic valves, shock absorbers, vibrators or devices for positioning and fixing workpieces, computer output devices that create a voltage dependent resistance such as used in virtual reality simulations (e.g. virtual reality simulations of medical surgery), and computer output devices that simulate a surface having a controllable texture such as a refreshable touch pad for reading Braille text.
The non-aqueous liquids useful as a dispersion medium for electro-viscous fluids may be, for example, hydrocarbons such as paraffins, olefins and aromatic hydrocarbons, Silicone oils such as polydimethylsiloxanes and liquid methyl phenyl siloxanes are also used. These may be used singly or as combinations of two or more types. The liquid dispersing medium is selected or formulated so that it is liquid over a fairly wide temperature ranges, usually at least -30.degree. C. (243.degree. K) to 150.degree. C. (423.degree. K). The viscosity of the dispersing medium is selected to be as low as praticable without excessive volatility because this enables a lower basic viscosity of the electro-viscous fluid. To avoid sedimentation of the dispersed phase, the dispersing medium and the dispersant should have a density that is approximately equal to each other. When this condition is not fulfilled either the dispersant floats to the surface of the fluid because it is less dense or it settles to the bottom of the fluid because it is more dense.
In many known electro-viscous fluids the disperse phase consists of inorganic solids such as silica gel, zeolites, alumina, titania, spherical particles obtained by the hydrolysis and condensation of metal alkoxides, polymers, substituted silicone resins and composite particles obtained by the condensation of ionic polymers with certain silicon compounds. In general, the effects of an electric field dependent viscosity is believed to be due to the presence of adsorbed water on the particles of the dispersed phase. Ions present in the adsorbed water thus align with the field and thus the particles repel one another with a force proportional to the applied electric field.
In addition to the density considerations required for the disperse phase of an electro-viscous material the particle size and the particle size distribution have an effect on the electro-viscous effect. Generally particles having an average particle diameter of from 0.2 to 30 micrometers and a particle distribution relative half-width value below 0.8. The relative half width value is usually defined to be the absolute half-width value of the distribution divided by the average particle diameter.